Before using a secure device, a user is often required to enter a character sequence known as a Personal Identification Number (PIN) to unlock the secure device. Some secure devices are produced without keypads on which a PIN may otherwise be entered. Accordingly, the user generally enters the PIN using a primary device (e.g., a mobile phone, a portable digital assistant, a personal computer) that is in communication with the secure device. Subsequent to a candidate PIN being entered on the primary device, the candidate PIN is transmitted between several different software and hardware components of the primary device. For example, the candidate PIN may be transmitted from a user interface (UI) component that has received the candidate PIN to an application that caused the UI component to be presented to the user. Furthermore, the UI component may transmit the candidate PIN to a kernel and the kernel may transmit the candidate PIN to the secure device via several more components. In some cases, the candidate PIN may be transmitted to the secure device as clear text; in other cases, the candidate PIN may be transmitted to the secure device in an encrypted form. An attacker, therefore, has multiple opportunities to intercept a transmission of the candidate PIN from component to component between the time at which the user enters the candidate PIN on the primary device and the time at which the candidate PIN is transmitted by the primary device to the secure device.